The need to develop efficient and robust seismic techniques for the assessment of lateral continuity (scale parameters) in the layered, heterogeneous media surrounding boreholes has long been recognized. Information about lateral continuity is assistive in planning the extraction of oil, gas, minerals, and other commodities from the area surrounding a borehole, and in the detection of cavities, tunnels and faults for geotechnical, environmental and engineering studies.
FIG. 1 shows three subsurface models and illustrates the problem of assessing lateral continuity in the immediate vicinity of an exploration well (Halderson and Golf-Racht, 1992). The three different subsurface models yield the same results when the vertical scale is assessed. The vertical scale may be observed using conventional techniques such as core sampling and borehole logs. However, the subsurface models differ significantly as far as lateral continuity of lithological units are concerned (horizontal scale length).
Resolution is the ability to distinguish separate features. Resolution is related to the minimum distance between 2 features that is required so that the features may be defined separately and not as one. The lateral resolution of conventional seismic methods for assessing lateral continuity is limited by the Fresnel Zone radius of the transmitted seismic wavefield. For techniques that use a surface seismic wavefield source, the result is that the minimum size of heterogeneities (e.g. oil reservoirs) that can be resolved increases with the depth of the exploration target being examined.
Well logs are physical measurements as a function of depth in a borehole. Examples include logs that record electrical, acoustic, nuclear and geotechnical measurements. These logs provide information about the formation probed by the borehole, commonly in the cm to dm range. However, it is impractical to drill sufficient boreholes to achieve resolutions in this range over the entire site to be explored.
Common techniques to assess lateral continuity of subsurface formations are 2D and 3D surface seismic methods, offset Vertical Seismic Profiling (“VSP”) techniques and cross-well seismic methods.
Surface seismic exploration methods are based on energy traveling from seismic sources (located at or near the earth's surface) down to where physical rock properties (density, compressional and shear modulus) change. There a portion of the seismic energy is reflected back to the surface where it is detected by sensors. Horizontal resolution in such surface seismic methods is governed by the Fresnel Zone, the portion of a horizontal reflector at a depth from which the reflected energy can reach a sensor within one-half wavelength of the first reflected energy. The width of the Fresnel Zone increases with the increasing depth of the reflector, thus limiting the lateral resolution of surface seismic methods.
Another way to look at the side of a borehole is offset VSP and walk away VSP techniques (using sensors in the borehole and surface seismic sources). Lateral resolution is limited by the width of the Fresnel Zone and limited azimuthal coverage by the surface seismic sources. VSP techniques rely on the separation of down-going and up-going wavefields recorded in the borehole.
Cross-well seismic methods investigate the region between two boreholes (sources are located in one borehole and receivers are located in the other borehole). This method overcomes limitations imposed by surface seismic sources; however, information about the lateral continuity of a formation is restricted to the plane between the boreholes. In addition, this method requires two or more boreholes.
A solution which addresses some or all of these needs or drawbacks is therefore desired.